The U.S. Space Shuttle Columbia broke apart in mid-air Saturday minutes before it was to return to Earth. The U.S. space agency, NASA, says none of the seven astronauts aboard survived.

Just 16 minutes before Columbia's scheduled landing in Florida, mission controllers lost all radar and voice contact with the shuttle as it passed high over north central Texas.

Dramatic video images from a Dallas television station shows pieces of the shuttle falling from the sky as it returned home after a 16-day scientific research mission. "Sadly, I think from the video that's available, it does not appear that there were any survivors," said Bill Readdy, a former astronaut who supervises NASA's human space flight programs. "At this point, I'd have to say it's too early to speculate about the exact cause," he added. "Obviously we're looking at all the data we have available."

NASA mobilized emergency rescue teams in the Dallas-Forth Worth area of northern Texas and warned residents in the area not to touch any of the debris.

Flags are being flown at half-staff at all space agency facilities. The flag at the White House also was lowered. NASA administrator Sean O'Keefe has established internal and independent external investigation boards to look into the cause of the disaster. "This is indeed a tragic day for the NASA family, for the families of the astronauts, and likewise tragic for the nation," he said.

17 years ago, the shuttle Challenger blew up 74 seconds after launch, but the Columbia disaster is the first time a shuttle has been lost returning from orbit since the program began 113 missions ago in 1981.

At the U.S. Naval War College in Newport, Rhode Island, space expert Joan Johnston-Freese says takeoff and landings are the most dangerous times for space shuttles. "That's when the maximum pressure and velocity occur," said Joan Johnston-Freese. "The shuttle lands as a large glider and control is always a challenge, but under those conditions of pressure and velocity, the shuttle is so super-heated at that point that it's a very volatile situation under the best of conditions."

NASA administrator O'Keefe honored the Columbia crew by saying they dedicated their lives to pushing the scientific challenges for all of us here on Earth. "I was here this morning with the families of the astronauts and their friends," said Sean O'Keefe. "It started out as a pretty happy morning, awaiting the landing of STS-107. We had highly anticipated their return because we couldn't wait to congratulate them."

Security was tighter than usual at the landing site, because the presence of Israel's first astronaut, Ilan Ramon, prompted NASA fears that he might be the target of a terrorist attack.

However, NASA says there is no indication that terrorism is involved in the shuttle loss.